The present invention relates to flat plate antennas.
Flat plate or low profile antennas such as planar inverted-F antennas (PIFA) are well known in the art. An example of a PIFA having an edge feed is shown in FIG. 1 of the accompanying drawings. The PIFA 100 comprises a flat conductive sheet 102 supported a height L.sub.1 above a reference voltage plane 104 such as a ground plane. The sheet 102 may be separated from ground plane 104 by an air dielectric, or supported by a solid dielectric. A corner 106 of the flat sheet 102 is coupled to ground via stub 108. A feed section 110 is coupled to an edge of the flat sheet 102 adjacent grounded corner 106 at feed point 112. Feed section 110 may comprise the inner conductor of a coaxial feed line having a dielectric inner 114, and an outer conductor which is coupled to the ground plane 104. The PIFA 100 forms a resonant circuit having capacitance and inductance per unit area. Feed point 112 is positioned on sheet 102 a distance L.sub.2 from corner 106 such that the impedance of the antenna 100 at that point matches the output impedance of the feed section, which is typically 50 ohms. The main mode of resonance for PIFA 100 is between the short circuit 106, and open circuit edge 116. Thus, the resonant frequency supported by PIFA 100 is dependent on the length of the sides of sheet 102, and to a lesser extent the distance L.sub.1 and thickness of sheet 102.
Planar inverted-F antennas have found particular applications in the radio telephone art where their high gain and omni-directional radiation patterns are particularly suitable. They are also suitable for applications where good frequency selectivity is required. Additionally, since the antennas are relatively small at typical radio telephone frequencies they can be incorporated within the housing of a radio telephone, thereby not interfering with the overall aesthetic appeal of the radio telephone and giving it a more attractive appearance than radio telephones having external antennas. By placing the antenna inside the housing of a radio telephone, the antenna is less likely to be damaged and therefore have a longer useful life. The PIFA lends itself to planar fabrication, and may suitably be fabricated on the printed circuit board typically used in a radio telephone to support the electronic circuitry. This lends itself to cheap manufacture.
However, PIFA are relatively narrowband devices, typically 3.5% bandwidth about a nominal centre frequency. Thus, they are unsuitable for wide band or multi-band applications.